The present invention relates to optical microphone/sensors. More particularly, the invention relates to fiber optic and solid waveguide microphone/sensors for sensing sounds in audio, ultra-sound and infra-sound ranges and for measuring distances to, and/or physical properties of, a medium according to U.S. Pat. No. 5,777,091 and U.S. patent application Ser. No. 09/037,137, the teachings of which are incorporated herein by reference.
In accordance with the teachings of U.S. Pat. No. 5,777,091 and U.S. patent application Ser. No. 09/037,137, an optical sensor consists of a source of light that produces the light used for measurements. One optical fiber or waveguide channels this light to the sensor""s optical head; after the light is reflected from the measuring medium, it passes through another optical fiber or waveguide to a light-intensity measuring means that measures the intensity of the returned light.
Microphone/sensors, especially those of the subject kind, are very sensitive to changes in atmospheric pressure. Such changes influence the sensitivity and accuracy of the microphone/sensors.
It is therefore a broad object of the present invention to overcome the shortcomings of the known type of optical microphone/sensors and to provide microphone/sensors which are not sensitive to changes in atmospheric pressure.
It is a further object of the present invention to provide a optical microphone/sensor made of non-metallic parts, rendering the microphone/sensor insensitive to electromagnetic fields.
In accordance with the present invention, there is provided an microphone/sensor, comprising: a housing closed at one end; a pair of optical waveguides each fixed at one end to said closed end of the housing, and extending within the housing towards the opposite end of the housing; a light source optically coupled to said one end of one of the optical waveguides; a light detector optically coupled to said one end of the other optical waveguide; a deformable membrane deformable by pressure waves closing the opposite end of said housing proximate to the opposite end of said pair of optical waveguides to form a sealed chamber with said closed one end of the housing; said membrane having an inner surface facing, but spaced from, said opposite ends of the optical waveguides, and an outer surface exposed to pressure wave in the atmosphere, such that said inner surface of the membrane influences light received by said other optical waveguide from said one optical waveguide in accordance with deformations of said membrane; and equalizing means for equalizing the pressure on the opposite sides of said membrane; characterized in that said equalizing means includes a capillary tube passing through the housing into said sealed chamber.
In the described preferred embodiments, the capillary tube is of a length and diameter that only small changes in atmospheric pressure resulting in frequency changes of less than 0.01 Hz influence the pressure within said sealed chamber.